This application claims the benefit of a priority under 35 USC 119 to French Patent Application No. 0105822 filed Apr. 30, 2001, the entire contents of which are hereby incorporated by reference.
The invention is directed to a method and apparatus for a radiographic image of an object obtained by a radiography apparatus having an anti-scatter grid.
The invention may be applied but not exclusively to mammography examinations, principally for the detection of microcalcifications inside a breast.
A radiography apparatus, as generally used in mammography, is equipped with an anti-scatter grid placed between the object to be X-rayed, in this case a breast, and a receiver of X-ray images. The anti-scatter grid comprises a series of plates spaced apart and directed toward the focal point of an X-ray beam emitted toward the object and an image receiver. Thus, the anti-scatter grid lets unscattered direct beams pass, while the scattered beams are absorbed by the plates.
The resolution of the image receiver is generally finer than the spacing between two plates, which is typically in the order of 0.3 mm. As a result, the plates are visualized on the radiographic image obtained. This visualization interferes with the mammography, as it renders more difficult the detection of microcalcifications, and principally microcalcifications associated with pathological lesions.
One known solution comprises displacing the grid during exposure, in rectilinear translation in its plane, that is, roughly perpendicular to the plates of the anti-scatter grid. Such a translation can be carried out solely in one direction, or alternatively in both directions. The image quality is thus improved, but still remains insufficient. Furthermore, the generation of an alternating motion is a relatively complex mechanical solution.
Patent FR 2,784,569 discloses a method for a radio-graphic image of an object obtained by a radiography apparatus equipped with an anti-scatter grid driven in translation according to a time displacement law. The time displacement low is a continuous curve presenting a point symmetry in relation to the point whose time coordinate is equal to half the duration of exposure and whose time variable spatial derivative presents two symmetrical linear portions in relation to an axis of symmetry passing through the middle of the range of displacement of the grid. This method proves satisfactory in case the instantaneous energy supplied by the emitter in the form of X-radiation is known with precision and is constant during the period of exposure. The quality of the image supplied by the receiver depends on the quantity of energy received in the form of X-radiation by the receiver upon the period of exposure. Known techniques do not make it possible to obtain an X-radiation source possessing an instantaneous energy level of the X-radiation emitted, determined with precision and constant. As a result, the plates of the anti-scatter grid are still visible to some extent.
An embodiment of the invention proposes eliminating the visible traces of the plates of the anti-scatter grid on the X-ray film as much as possible. An embodiment of the invention proposes obtaining an improvement of image quality with mechanically simple means for displacement of the anti-scatter grid.
According to an embodiment, the invention is a method and apparatus for a radiographic image of an object obtained by a radiography apparatus containing an X-ray emitter, in which an anti-scatter grid placed between the object and a radiographic image receiver is displaced, on rectilinear translation in its plane, on shooting of images, between a starting position and an arrival position, according to a predefined time displacement law, at a greater rate of displacement in proximity to a starting position and to an arrival position than between those two positions. A measurement of the energy received in the form of X-radiation by the receiver is used to modify the rate of displacement of the grid at every instant. Thus, the displacement of the anti-scatter grid can be adjusted to the energy received by the receiver, so that, during the exposure time, each part of the receiver has been exposed to substantially the same quantity of X-radiation energy, apart from the rays absorbed by the object studied itself. The energy received by the receiver is measured from the energy emitted by the X-ray emitter. The energy received in the form of X-rays by the receiver is measured from the energy coming to the receiver.
An embodiment of the invention is directed to a radiographic imaging device, comprising means for providing an X-ray emitter, means for receiving the X-rays after having crossed an organ to be studied, an anti-scatter grid movable in translation in a plane roughly perpendicular to the X-rays, means for displacement capable of producing the displacement of the anti-scatter grid in its plane on shooting of the image, between a starting position and an arrival position, according to a predefined time displacement law, means for computing, means for measurement of the energy received by the means for receiving in the form of X-rays, and means for modifying the predefined time displacement law. The means for computing is connected to the means for measurement. The means for computing is capable of emitting a control signal to the means for displacement. The data received by the means for computing from the means for measurement can make a correction to the speed of the anti-scatter grid. According to an embodiment, the means for measurement is disposed on an energy supply link of the means for providing an emitter, in order to measure the energy actually emitted by the emitter. According to an embodiment, the means of measurement is disposed on the means for receiving in order to measure the energy actually received by the receiver.